Alpha-olefins in the range of 4-40 carbon atoms are known to be prepared by wax cracking, paraffin dehydrogenation and dehydration of alcohols. However, these processes suffer from the drawback of low conversion, low product purity of specific alpha-olefins and high energy costs.
Other widely used processes to prepare alpha-olefins are based on ethylene chain growth and displacement reactions [Ziegler]. These processes utilize trialkyl aluminum, particularly triethylaluminum at high temperatures [170-200.degree. C.] and pressures [140 atm]. A particular drawback of such processes is the difficulty in the separation of lighter and higher olefins and the formation of internal and vinylidene olefins.
Several patents known hereto before disclose the catalytic low temperature oligomerization of ethylene based on titanium halide [e.g. TiCl.sub.4 ], alkyl aluminum halide [e.g. EtAlCl.sub.2 ] [Ziegler-Natta] modified by phosphines [e.g. nBu.sub.3 P]. The selectivity of alpha-olefins in these cases is affected by copolymerization reactions yielding undesirable branched olefins.
British Patent No. 787,438, issued on Dec. 11, 1957 describes the preparation of lower alpha-olefins with less than 6 carbon atoms which are prepared by utilizing a catalyst prepared by combining titanium tetraalkoxide and triethylaluminum. This process involves moderate ethylene conversions.
Low temperature oligomerization by halogenated titanium alkoxides yielding alpha olefins up to C.sub.50 is disclosed in German Patent No. 1,924,427, issued on Apr. 23, 1970. Another class of catalysts which have been utilized are organoaluminum-free compounds [non-Ziegler] [e.g. Ni(COD).sub.2 or nickel ylides] which are used to convert ethylene to higher oligomers of even carbon numbers [e.g. C.sub.4 -C.sub.20+ ] which are linear in character. The non-Ziegler route using bis [cyclooctadiene] nickel [o] or its complexes with fluorinated compounds [e.g. hexafluoroacetyl acetone] is described in the U.S. Pat. No. 3,644,564 issued on Feb. 22, 1972 and commercialized in the Shell Higher Olefins Process [SHOP].
The primary objective of the present invention is to provide a novel process for producing linear alpha-olefins using a novel catalyst system.
The process results in the production of linear alpha-olefins having 4-36 carbon atoms and particularly, is highly selective for the more useful alpha-olefins in the range of C.sub.4 -C.sub.20.
Another aspect of the invention involves the high rate of conversion of ethylene to low molecular weight product olefins under controlled reaction conditions.